The first step in understanding a transmission system is to define the basic broadband transmission unit. For analog systems, this would be a channel group, which prior to transmission is typically multiplexed into super-groups and mastergroups. For digital transmission, the basic unit is the DS1 signal. The DS1 signal developed by a digital channel bank (e.g., the D-3 Channel Bank, as disclosed in U.S. Pat. No. 4,059,731, issued Nov. 22, 1977 to J. H. Green and J. E. Landry) and transmitted over a T-1 transmission line (1.544 megabits per second) is, at present, the workhorse of the Bell System digital transmission network.
The format of the DS1 digital signal consists of 24 eight-bit words and one framing bit for a total of 193 bits per frame. The 24 words typically represent 24 separate and distinct messages deposited in 24 separate and distinct channels. The words are PCM (pulse code modulation) encoded and the least significant bit (i.e., eighth bit) of a channel is periodically dedicated (every sixth frame) for signaling purposes.
Over the past several years there has been increasing interest in achieving a more efficient digital encoding. For an evolving digital network, a most interesting application is the possible replacement of the 64,000 bit-per-second (bps) PCM signal (8 bits per channel, repeated at an 8 kHz rate) for telephony. The reason, of course, is to achieve bandwidth compression, and thus a concomitant increase in transmission capacity. To this end, the patent application of D. W. Petr, Ser. No. 343,355 filed Jan. 27, 1982 discloses an efficient, robust, bit compression algorithm. In accordance with the Petr invention, each 64 Kbps signal is converted or compressed to a 32 Kbps signal thereby doubling the capacity of a T1 line, for example.
Using the Petr algorithm, a pair of DS1 digroups (digital groups) can be bit compressed and multiplexed together to form a digroup of twice the regular (T1) transmission capacity, but with the very same bit rate (1.544 Mbps). However, there remains the problem of signaling. With a DS1 digital signal the least significant bit of a channel is periodically preempted for signaling purposes with little degradation in performance. But, to periodically preempt one of the four bits of a sub-rate (32 Kbps) channel for signaling would significantly impair performance because the necessary coordination between the coder and decoder would be deleteriously affected. Alternatively, two or more subrate channels might be dedicated for signaling purposes. This, however, would reduce the number of channels available for message transmission purposes. Accordingly, while bit compression is potentially most advantageous, it presents its own problems--particularly with signaling transmission.